Various organic membranes and inorganic membranes have conventionally been proposed as separation membranes. However, organic membranes have low solvent resistance and heat resistance, although they are inexpensive and excellent in moldability. As opposed to organic membranes, inorganic membranes, such as ceramic membranes, have excellent solvent resistance and heat resistance; however, they have problems of high cost and difficulty in molding.
Accordingly, carbon membranes, which are inorganic membranes, but have excellent moldability and are inexpensive, have recently attracted attention. Hollow fiber carbon membranes have pores of a size that allows gas separation, and exhibit excellent gas separation performance among various inorganic membranes. Further, hollow fiber carbon membranes can be used in an environment for which heat resistance against a temperature as high as about 70 to 150° C., at which organic membranes cannot be used, and chemical resistance are required. Accordingly, the practical use of hollow fiber carbon membranes is highly expected. Moreover, hollow fiber membranes have excellent pressure resistance, a large membrane area per unit volume, and capability of producing compact separation membrane modules.
Conventionally proposed hollow fiber carbon membranes are those produced, using as a raw material, for example, a resin obtained by sulfonating polyphenylene oxide (Patent Documents 1 and 2), and aromatic polyimide (Patent Document 3).
However, sulfonated polyphenylene oxide itself is not a versatile material, and therefore requires a synthesis process to sulfonate polyphenylene oxide. On the other hand, the synthesis of aromatic polyimide requires a reaction in an organic solvent; however, since it is difficult to ensure the solubility in the organic solvent, a special production method is necessary. Thus, carbon membranes produced using sulfonated polyphenylene oxide or aromatic polyimide as a raw material have problems of high membrane cost, because raw materials are expensive, and the preparation of raw materials and the membrane-forming process are complicated.
In contrast, a carbon membrane produced using inexpensive polyphenylene oxide as a raw material is also proposed (Patent Document 4). However, separation properties are low only with polyphenylene oxide; therefore, ensuring separation properties requires a complicated structure in which a sulfonated polyphenylene oxide resin is laminated on a polyphenylene oxide membrane, followed by calcination treatment, and the production process becomes complicated. Accordingly, there is a problem of high cost, despite the use of the inexpensive raw material.
In general, when any organic raw material is used, carbon membranes require spinning of hollow fibers, followed by two-step heating comprising, an “infusibilization treatment” in which heating is performed at 250 to 350° C. in the air, and a subsequent “carbonization treatment” in which heating is performed at 600 to 800° C. in an inert atmosphere or under vacuum.
Therefore, in order to produce carbon membrane hollow fiber membranes with excellent cost performance, there is a demand for a production method in which a hollow fiber is spun using inexpensive organic matter materials, and without adopting complicated processes, a two-step heating process comprising an infusibilization treatment step and a carbonization treatment step is performed as a main process. However, polyphenylene oxide, which is an inexpensive material, is a thermoplastic resin, and passes through about 220° C., which is the melting temperature of polyphenylene oxide, at the infusibilization step in which heating is performed at 250 to 350° C.; therefore, the hollow fiber shape formed by spinning is impaired due to the melting of the polyphenylene oxide, and crushing and breakage of the hollow fibers, fusion between the hollow fibers, etc., occur. Consequently, the target hollow fiber membrane in a strain-free hollow shape may not be obtained, and desired gas separation performance may not be obtained.
Thus, in conventionally known production methods, when polyphenylene oxide was used alone as a membrane material, and when a hollow fiber carbon membrane was produced through an infusibilization step and a carbonization step, it was difficult to obtain excellent moldability and desired gas separation performance in some cases.